Nickel-based superalloy having very high resistance to hot-corrosion for monocrystalline blades of industrial turbines

ABSTRACT

Nickel-based superalloy, suitable for monocrystalline solidification, having the following composition by weight:  
                                                       Co:   4.75       to   5.25%         Cr:   15.5   to   16.5%         Mo:   0.8   to    1.2%         W:   3.75       to   4.25%         Al:   3.75       to   4.25%         Ti:   1.75       to   2.25%         Ta:   4.75       to   5.25%         C:   0.006   to    0.04%         B:       ≦    0.01%         Zr:       ≦    0.01%         Hf:       ≦    1%         Nb:       ≦    1%                   Ni and any impurities: complement to 100%.

TECHNICAL FIELD

[0001] The invention relates to a nickel-based superalloy which isadapted to the manufacture of fixed and movable monocrystalline bladesof industrial gas turbines by directional solidification.

BACKGROUND OF THE INVENTION

[0002] Nickel-based superalloys are the most high-performance materialsused today in the manufacture of movable and fixed blades of industrialgas turbines. The two principal features required until now of thesealloys for those specific applications have been good resistance tocreep at temperatures of up to 850° C. and very good resistance tohot-corrosion. Some reference alloys currently used in this field aredesignated IN738, IN939 and IN792.

[0003] Blades manufactured using those reference alloys are produced byconventional casting using the lost-wax process and have apolycrystalline structure, that is to say, they are constituted by thejuxtaposition of crystals which are orientated in a random mannerrelative to each other and which are called grains. Those grains arethemselves constituted by an austenitic gamma (γ) matrix based onnickel, in which hardening particles of the gamma prime (γ′) phase aredispersed whose base is the intermetallic compound Ni₃Al. This specificstructure of the grains gives those alloys a high level of creepresistance up to temperatures in the order of 850° C., which ensures thelongevity of the blades, for which service lives of from 50,000 to100,000 hours are generally sought. The chemical composition of alloysIN939, IN738 and IN792 has further been determined to give themexcellent resistance to the combustion gas environment, in particular inrespect of hot-corrosion, a phenomenon which is particularly aggressivein the case of industrial gas turbines. Significant additions of chrome,typically of from 12 to 22% by weight, are thus necessary to give thosealloys the necessary resistance to hot-corrosion for the applicationsconcerned. From the point of view of resistance to creep, the order ofthe alloys is: IN939<IN738<IN792. From the point of view of resistanceto hot-corrosion, the order is the reverse, that is: IN792<IN738<IN939.

[0004] In order to improve the performance of industrial gas turbines interms of output and consumption, one method consists in increasing thetemperature of the gases at the turbine inlet. This consequently makesit necessary to be able to provide alloys for turbine blades which cantolerate operating temperatures which are higher and higher, whilstretaining the same mechanical features, in particular in terms of creep,in order to be able to achieve the same service lives.

[0005] The same type of problem has been posed in the past in the caseof gas turbines for turbo-jets and turbo-engines for aeronauticalapplications. In this case, the selected solution consisted in changingfrom blades, known as polycrystalline blades, which are produced byconventional casting to blades, known as monocrystalline blades, that isto say, which are constituted by a single metallurgical grain.

[0006] Those monocrystalline blades are manufactured by directionalsolidification with lost-wax casting. The elimination of grainboundaries, which are preferential locations for creep deformation atelevated temperature, has allowed the performance of nickel-basedsuperalloys to be increased spectacularly. Furthermore, the process ofmonocrystalline solidification allows the preferred orientation ofgrowth of the monocrystalline component to be selected and, in thatmanner, the orientation <001> which is optimum from the point of view ofresistance to creep and thermal fatigue to be chosen, those two types ofmechanical stress being the most disadvantageous for turbine blades.However, the chemical superalloy compounds developed for monocrystallineturbine blades for aeronautical applications are not suitable for bladesfor terrestrial or marine applications, known as industrialapplications. Those alloys are determined in order to promote theirmechanical resistance up to temperatures greater than 1100° C., and thisto the detriment of their resistance to hot-corrosion. In that manner,the concentration of chrome of the superalloys for aeronauticalmonocrystalline turbine blades is generally less than 8% by weight,which allows volume fractions of the γ′ phase in the order of 70% to beachieved, which levels are advantageous for resistance to creep atelevated temperature.

[0007] A nickel-based superalloy which is rich in chrome and which issuitable for the monocrystalline solidification of components ofindustrial gas turbines is known by the designation SC16 and isdescribed in FR 2 643 085 A. Its concentration of chrome is equivalentto 16% by weight. The features concerning the creep resistance of alloySC16 are such that the alloy provides, relative to the polycrystallinereference alloy IN738, an increase in operating temperature ranging fromapproximately 30° C. (830° C. instead of 800° C.) to approximately 50°C. (950° C. instead of 900° C.). Comparative tests for cyclicalcorrosion at 850° C. in air at atmospheric pressure with Na₂SO₄contamination showed that the resistance to hot-corrosion of alloy SC16was at least equivalent to that of the reference polycrystalline alloyIN738.

[0008] Hot-corrosion tests have been carried out on alloy SC16 by themanufacturers of industrial turbines on their own test benches. In verysevere environments, which are representative of extreme operatingconditions, it has been shown that the resistance to hot-corrosion ofthat alloy remained inferior to that of alloy IN738.

[0009] Furthermore, the increasing demand from those manufacturers foran increase in the operating temperature of gas turbines gives rise tothe need for superalloys for blades to have a resistance to creep whichis increased still further.

SUMMARY OF THE INVENTION

[0010] The problem addressed by the invention is to provide anickel-based superalloy having a resistance to hot-corrosion in theaggressive combustion gas environment of industrial gas turbines whichis at least equivalent to that of reference polycrystalline superalloyIN738, and having a resistance to creep which is greater than or equalto that of reference alloy IN792 within a temperature range of up to950° C.

[0011] This superalloy must in particular be suitable for manufacture offixed and movable monocrystalline blades having large dimensions (up toseveral tens of centimetres in height) of industrial gas turbines bydirectional solidification.

[0012] Furthermore, this superalloy must demonstrate goodmicro-structural stability in respect of the precipitation of fragileintermetallic phases which are rich in chrome when maintained forsustained periods at elevated temperature.

[0013] More specifically, an alloy compound is sought which ensures:

[0014] optimised resistance to hot-corrosion, in any case at leastequivalent to that of reference polycrystalline superalloy IN738, andthis in an environment which is representative of that for combustiongases of industrial turbines;

[0015] a maximum volume fraction of hardening precipitates of the γ′phase in order to promote resistance to creep at elevated temperature;

[0016] resistance to creep up to 950° C. which is at least equivalent tothat of reference polycrystalline alloy IN792;

[0017] a tendency to homogeneity by completely placing in solutionparticles of the γ′ phase, including the γ/γ′ eutectic phases;

[0018] the absence of precipitation of fragile intermetallic phaseswhich are rich in chrome, starting from the γ matrix, when maintainedfor sustained periods at elevated temperature;

[0019] a density which is less than 8.4 g.cm⁻³ in order to minimise themass of the monocrystalline blades and, consequently, to limit thecentrifugal stress acting on the blades and on the turbine disc to whichthey are fixed;

[0020] a good tendency to monocrystalline solidification of turbineblades whose height can reach several tens of centimetres and the massseveral kilograms.

[0021] The superalloy according to the invention, which is suitable formonocrystalline solidification, has the following composition by weight:Co: 4.75 to 5.25% Cr: 15.5 to 16.5% Mo: 0.8 to  1.2% W: 3.75 to 4.25%Al: 3.75 to 4.25% Ti: 1.75 to 2.25% Ta: 4.75 to 5.25% C: 0.006 to  0.04%B: ≦  0.01% Zr: ≦  0.01% Hf: ≦  1% Nb: ≦  1% Ni and any impurities:complement to 100%.

[0022] The alloy according to the invention is an excellent compromisebetween resistance to creep and resistance to hot-corrosion. It is forthe manufacture of monocrystalline components, that is to say,components which comprise a single metallurgical grain. This specificstructure is obtained, for example, by means of a conventionaldirectional solidification process at a thermal gradient, using ahelical or chicane-like device for selecting a grain, or a monocrystalnucleus.

[0023] The invention also relates to an industrial turbine blade whichis produced by monocrystalline solidification of the above superalloy.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The features and advantages of the invention will be set forth ingreater detail in the description below with reference to the appendeddrawings.

[0025] FIGS. 1 to 4 are graphs illustrating the properties of differentsuperalloys.

DETAILED DESCRIPTION

[0026] An alloy according to the invention designated SCA425 has beenproduced with reference to the nominal composition listed in Table I. Inthis Table, the nominal concentrations of major elements of referencealloys IN939, IN738, IN792 and SC16 are also listed. TABLE IConcentrations by weight of major elements (%) Alloy Ni Co Cr Mo W Al TiTa Nb IN939 Base 19 22.5 — 2 1.9 3.7 1.4 1 IN738 Base  8.5 16 1.7 2.63.4 3.4 1.7 0.9 IN792 Base  9 12.4 1.9 3.8 3.1 4.5 3.9 — SC16 Base — 163 — 3.5 3.5 3.5 — SCA425 Base  5 16 1 4 4 2 5 —

[0027] Chrome has an advantageous and dominant effect on the resistanceto hot-corrosion of nickel-based superalloys. Thus, tests have shownthat a concentration in the order of 16% by weight was necessary in thealloy of the invention in order to obtain resistance to hot-corrosionthat is equivalent to that of reference alloy IN738 under the conditionsfor hot-corrosion tests described below, which conditions arerepresentative of the environment created by combustion gases of someindustrial turbines. Chrome also contributes to the hardening of the γmatrix in which this element is preferentially distributed.

[0028] Molybdenum greatly hardens the γ matrix in which the element ispreferentially distributed. The quantity of molybdenum which can beintroduced to the alloy is limited, however, because the element has adisadvantageous effect on the resistance to hot-corrosion ofnickel-based superalloys. A concentration in the order of 1% by weightin the alloy of the invention is not detrimental to the corrosionresistance and contributes significantly to its hardening.

[0029] Cobalt also contributes to the hardening in the form of a solidsolution of the γ matrix. The concentration of cobalt has an effect onthe dissolution temperature of the γ′ hardening phase (γ′ solvustemperature). Thus, it is advantageous to increase the concentration ofcobalt in order to decrease the solvus temperature of the γ′ phase andto facilitate the homogenising of the alloy by means of heat treatmentwithout any risk of causing melting to start. Furthermore, it can alsobe advantageous to reduce the concentration of cobalt in order toincrease the solvus temperature of the γ′ phase and to benefit in thatmanner from greater stability of the γ′ phase at elevated temperature,which promotes resistance to creep. A concentration in the order of 5%by weight of cobalt in the alloy of the invention leads to an optimumcompromise between a good capacity for homogenising and good resistanceto creep.

[0030] Tungsten, whose concentration is in the order of 4% by weight inthe alloy of the invention, is distributed in a substantially equalmanner between the γ and γ phases and, in that manner, contributes tothe respective hardening processes thereof. Its concentration in thealloy is, however, limited because the element is heavy and has anegative effect on the resistance to hot-corrosion.

[0031] The concentration of aluminium is in the order of 4% by weight inthe alloy of the invention. The presence of the element causes theprecipitation of the γ′ hardening phase. Aluminium also promotesresistance to oxidation. The elements titanium and tantalum are added tothe alloy of the invention in order to reinforce the γ′ phase in whichthey are substituted for the element aluminium. The respectiveconcentrations of those two elements in the alloy of the invention arein the order of 2% by weight for titanium and 5% by weight for tantalum.Under the conditions for hot-corrosion tests described below,corresponding to the intended application, tests showed that thepresence of tantalum was more favourable to the resistance tohot-corrosion than was the case with titanium. However, tantalum isheavier than titanium, which is disadvantageous in respect of thedensity of the alloy. The total of the concentrations of tantalum,titanium and aluminium roughly determines the volume fraction of the γ′hardening phase. The concentrations of those three elements have beenadjusted in order to optimise the volume fraction of the γ′ phase, whilekeeping the γ and γ′ phases stable when maintained for long periods atelevated temperature, and taking into consideration the fact that theconcentration of chrome has been fixed at approximately 16% by weight inorder to achieve the desired resistance to corrosion.

[0032] Alloy SCA425 has been produced in the form of monocrystals havingorientation <001>. The density of that alloy has been measured and foundto be equal to 8.36 g.cm⁻³.

[0033] After directional solidification, the alloy is substantiallyconstituted by two phases: the austenitic γ matrix, which is a solidnickel-based solution, and the γ′ phase, which is an intermetalliccompound whose basic formula is Ni₃Al and which precipitates mainlywithin the γ matrix in the form of fine particles measuring less than 1micrometre during cooling to the solid state. Contrary to what isgenerally found in monocrystalline superalloys for turbine blades, alloySCA425 does not contain any interdentritic solid particles of the γ′phase resulting from a eutectic transformation of the residual liquidonce solidification has ended.

[0034] Alloy SCA425 underwent homogenising heat treatment at atemperature of 1285° C. for 3 hours with cooling in air. Thistemperature is higher than the solvus temperature of the γ′ phase(dissolution temperature of the precipitates of the γ′ phase), which is1198° C., and less than the solidus temperature, which is 1300° C. Thetreatment is intended to dissolve all of the precipitates of the γ′phase, whose distribution of sizes is very wide in the coarse state ofdirectional solidification, and to reduce the chemical heterogeneitieswhich are associated with the dendritic solidification structure.

[0035] The interval between the γ′ solvus temperature of the alloySCA425 and its solidus temperature is very large, which allows readyapplication of the homogenising treatment without any risk of meltingand with the certainty of obtaining a homogeneous microstructure whichallows optimised resistance to creep.

[0036] The cooling which follows the homogenising treatment describedabove was carried out by hardening in air. In practice, the rate of thiscooling must be so high that the size of the particles precipitatedduring the cooling operation is less than 500 nm.

[0037] The homogenising heat treatment procedure which has just beendescribed is an example which allows the intended result to be achieved,that is to say, a homogeneous distribution of fine particles of the γ′phase whose size is no greater than 500 nm. This does not exclude thepossibility of obtaining a similar result by using a different treatmenttemperature provided that the temperature lies within the rangeseparating the γ′ solvus temperature and the solidus temperature.

[0038] Alloy SCA425 was tested after undergoing a homogenising treatmentas described above, then two annealing treatments which allow the sizeand the volume fraction of the precipitates of the γ′ phase to bestabilised. A first annealing treatment consisted in heating the alloyto 1100° C. for 4 hours with cooling in air, which leads tostabilisation of the size of the precipitates of the γ′ phase. A secondannealing treatment at 850° C. for 24 hours, followed by cooling in air,allows the volume fraction of the γ′ phase to be optimised. This volumefraction of the γ′ phase is estimated at 50% in alloy SCA425. Once allof the heat treatments are completed, the majority of the γ′ phase hasbeen precipitated in the form of cuboid particles whose size is between200 and 500 nm. A small fraction of fine particles of the phase γ′ whosesize does not exceed 50 nm is present between the large precipitates.

[0039] Hot-corrosion tests were carried out at different temperatures onalloy SCA425 using the following procedure: samples are partiallyimmersed in a container containing a mixture of combustion residueswhose composition by weight is as follows: 4.3% Na₂SO₄+22.7% CaSO₄+22.3%Fe₂O₃+20.6% ZnSO₄+10.4% K₂SO₄+2.8% MgO+6.5% Al₂O₃+10.4% SiO₂. A mixtureof air+0.15% SO₂ by volume passes through the mixture of combustionresidues at a rate of 6 litres per hour. The mixture of combustionresidues is renewed every 500 hours. This environment is representativeof the very aggressive environment of combustion gases for someindustrial turbines. For comparison purposes, samples of alloys IN738,IN939, IN792 and SC16 were tested at the same time.

[0040] The samples were cut into sections and the depth of metaldestroyed by the corrosion phenomenon was measured. The graphs in FIGS.1 to 3 show the mean depths of penetration of the corrosion for thedifferent alloys at 700° C., 800° C. and 850° C., respectively, as afunction of the test duration. The resistance to corrosion is evenbetter since the depth of penetration is low. At 700° C. and 800° C.,the alloy SCA425 demonstrates a resistance to corrosion equivalent tothat of alloy IN738 and better than that of alloy SC16. At 850° C., theresistance to corrosion of alloy SCA425 is comparable to that ofreference alloys IN738 and IN939.

[0041] Tests for creep under tensile stress were carried out on machinedtest pieces in monocrystalline bars of orientation <001>. The bars werehomogenised beforehand then annealed according to the proceduresdescribed above. Values for rupture times obtained at 750, 850 and 950°C. for different levels of stress applied are listed in Table II.

[0042] Table II: Service lives with creep of alloy SCA425 Temperature (°C.) Stress (MPa) Rupture time (h) 750 650   216/321.1 750 575  984 850400 201/276 850 300 2121/2945/3220 850 250 6161 950 250 73/76 950 200261/291 950 180  578 950 160 1098 950 140 2109 950 120 3872

[0043] The graph in FIG. 4 allows a comparison of the rupture times withcreep obtained for alloys SCA425, IN792 and SC16. The stress applied isplotted on the abscissa. The value of the Larson-Miller parameter ismarked on the ordinate. This parameter is the given by the formulaP=T(20+log t)×10⁻³, where T is the creep temperature in Kelvin and t isthe rupture time in hours. This graph shows that the creep resistance ofalloy SCA425 is at least equivalent to that of alloy IN792, which is thestipulated objective, and greater than that reference alloy SC16.

[0044] The inspection of the microstructure of the test pieces of alloySCA425 at the end of the creep tests demonstrated the absence ofprecipitation of fragile intermetallic particles which are rich inchrome and which are capable of appearing when maintained for sustainedperiods at elevated temperature in nickel-based superalloys where the γmatrix is over-saturated with additive elements.

[0045] Manufacturing tests on monocrystalline components of super-alloySCA425 demonstrated that it was possible to cast a large range ofcomponents whose mass can range from a few grammes to more than 10 kg,with various levels of complexity. The growth of components according tothe crystallographic orientation <001> is promoted and dominant and thepresence of grains that are orientated in a random manner is minimised.The liquid metal is stable in the sense that it does not react with thematerials commonly used in the manufacture of moulds. The phenomenon ofrecrystallisation which can occur during homogenising treatment atelevated temperature is absent in the case of alloy SCA425.

We claim:
 1. A nickel-based superalloy, suitable for monocrystallinesolidification, characterised in that its composition by weight is asfollows: Co: 4.75 to 5.25% Cr: 15.5 to 16.5% Mo: 0.8 to  1.2% W: 3.75 to4.25% Al: 3.75 to 4.25% Ti: 1.75 to 2.25% Ta: 4.75 to 5.25% C: 0.006 to 0.04% B: ≦  0.01% Zr: ≦  0.01% Hf: ≦  1% Nb: ≦  1% Ni and anyimpurities: complement to 100%.


2. Industrial turbine blade produced by monocrystalline solidificationof a superalloy according to claim 1.